The invention relates to a device for assembling a pneumatic booster of a motor vehicle braking device. More precisely, the invention relates to a device making it possible to fixedly attach a cover and a cylinder of a pneumatic booster. The invention also relates to a method making it possible to assemble the cylinder and the cover by using the device of the invention. The invention finally relates to a pneumatic booster.
One object of the invention is to provide a booster that can be used safely in a braking device. Another object of the invention is to prevent a disassembly of the booster during its use, irrespective of the thickness of a metal sheet forming the booster. An additional object of the invention is to provide a booster that is lightweight, without its strength being affected.
In a motor vehicle braking device, a booster may be mounted between a brake control and a master cylinder. The pneumatic booster has the role of amplifying a force at the brake control, so that a hydraulic pressure in the master cylinder is greater.
A booster may be furnished with a generally cylindrical-shaped casing. The casing is formed of a cylinder and a cover. The cylinder and the cover are fixedly attached to one another. A front chamber and a rear chamber are arranged in an internal volume of the casing. The front chamber is directed toward the master cylinder, and has a variable volume. The rear chamber, equally with a variable volume, is directed toward the brake control. The front chamber is separated from the rear chamber by a movable partition. The movable partition is formed by a sealed and flexible membrane and by a rigid skirt plate. The front chamber is connected pneumatically to a vacuum source. The rear chamber is connected pneumatically, in a manner controlled by a valve, to a source of propelling fluid. A brake control actuates a control rod of the booster. The actuation of the rod controls the opening of the valve and an inflow of fluid into the rear chamber. The change of pressure that results therefrom causes a movement of the rigid skirt plate.
During braking, wrenching forces exerted at the location of the booster are extremely great. Specifically, the chambers of the booster are subjected to sudden changes of pressure. In particular, during braking, air is allowed into the rear chamber. This inlet of air propels the movable partition toward the front chamber. Since the casing of the booster is made of two originally independent parts, it is essential that these two parts are firmly fixed together. It is therefore necessary that the contact between the cover and the cylinder is sufficient so that, during braking and more precisely during the propulsion of the skirt plate, the cylinder is not wrenched away from the cover.
In order to ensure that the cover is properly attached to the cylinder, spot swaging is currently carried out on an external periphery of the walls of the cover and the cylinder. “Spot swaging” means swaging at localized points. In order to hold the cylinder firmly onto the cover, these swaging points are evenly distributed over the whole external periphery of the casing. Hitherto, such an assembly has been sufficient.
However, for some time, the search has been on to reduce the weight of the booster. Specifically, the booster and more precisely the booster casing is formed of metal sheet walls. A thickness of the metal sheet and a shape of the booster therefore have an effect on its weight. The search in particular is to reduce the weight of the booster by reducing the thickness of the metal sheet forming the casing. The thickness and the shape of the booster have been modified in order to obtain a minimum weight. “Minimum weight” means the lowest weight of the booster at which the same rigidity and the same resistance to wrenching as a conventional booster is obtained.
However, the breaking strength at the location of the join between the cover and the cylinder is reduced. A spot swaging of the cover with the cylinder therefore does not provide full satisfaction when the thickness of the metal sheet is greatly reduced.